The physical downlink control channel (PDCCH) is used to carry downlink control information (DCI), required for reception of physical downlink control shared channel (PDSCH), and for scheduling grants enabling transmission on physical uplink shared channel (PUSCH). The PDCCH is transmitted in control channel region occupying a few Orthogonal Frequency Division Multiplexing (OFDM) symbols ranged from 1 to 3 at the beginning of the downlink sub-frame. Remaining part of the sub-frame is called data region. The data region is used for PDSCH transmission.
In order to allow for a simple processing of control channels in the receiver and simple multiplexing at the transmitter, the mapping of PDCCHs to resource element (RE) is subject to a certain structure. The structure is based on so-called control channel elements (CCE). CCE denote a set of 36 useful resource elements. The mapping of CCEs, which are logical units, to resource elements, which are physical units, is a function of the cell-id. The number of CCEs, i.e. one, two, four or eight required for a certain PDCCH, depends on the payload size of downlink control information and the channel coding rate. The number of CCEs used for a PDCCH is also referred to as the aggregation level. In practice blind decoding of the control channel is done on search spaces considering CCE as the processing unit. More precisely, a search space is a set of candidate control channels formed by CCEs on a given aggregation level, which the terminal is supposed to attempt to blindly decode.
One specific user terminal can be addressed only if the control information is transmitted on a PDCCH in the terminal's search space. Therefore, each user terminal in the system has its own search space known as a user specific search space (UE-SS). User specific search spaces in 3GPP Rel-8 are defined without explicit signalling. Starting position of UE-specific search space is a logical entity which is derived as a function of terminal identity and the sub-frame number.
In some situations, a group of user terminals in the system are addressed. This can happen for example for paging, sending system information and RACH procedure at the beginning of communication (initial step of communication). In order to be able to address all the user terminals, a common search space (CSS) is defined in 3GPP LTE. All the user terminals search in this common search space to decode related control information. The common search space is predefined and it consists of first 16 CCEs of the search space. This means that the starting position of common search space is set to zero. The aggregation level used for common search space is limited to four or eight.
It has been widely recognized that the control channel region for LTE PDCCH might be insufficient for future development scenarios where a significant increase of the number of users is expected. In order to increase the capacity of control channels a UE-specific control channel called enhanced PDCCH (ePDCCH) is introduced. The ePDCCH is allocated in the data region, and the ePDCCH can take advantage of some features which was reserved previously for PDSCH transmission, such as frequency selective scheduling to avoid interference and usage of reference signals which up to now have been used for PDSCH transmission only.
Enhanced control channel is supposed to be scheduled in data region as PDSCH with granularity of one PRB. Scheduling denotes dynamic or semi-static allocation of PRBs by a NodeB (base station). In the case of dynamic allocation, feedback information received from a UE indicates the suitable PRBs to use for sending related control information. More precisely, the ePDCCH is sent over a selected PRB and the selected PRB can change from one subframe to other based on received feedback information from the UE. In the case of semi-static allocation, selected PRB remains the same for one or more consecutive sub-frames and is changed for example when there is a handover or a newly received allocation. This is similar to scheduling of PDSCH in the downlink.
Definition of the basic logical unit of control channel called enhanced CCE (eCCE) is still an open issue. The eCCE definition will impact on the multiplexing of eCCEs in one (or several) PRB pair(s) and search space design. However, the eCCEs will contain a number of useful resource elements as in legacy PDCCH.
Another open topic is to decide if UE-specific search space related to enhanced control channel is predefined as it is the case for PDCCH or it is signalled to the UE and is changing from one sub-frame to other. The decision concerning the starting position of UE-specific search space is also in hold. This information could be based on UE-id or signalled to the UE. Whatever decided the definition of search space as a set of candidate control channels formed by eCCEs on a given aggregation level which the terminal is supposed to attempt to blindly decode remains valid.
Common search space related to enhanced control channel (if there is any) will anyhow occupy predefined resource elements eCCEs. Those predefined resource elements might occupy non consecutive PRB pairs.
The eCCEs are mapped in to one (or several) PRB pair(s) in presence of different reference signals. In LTE system, various reference signals are used to perform measurement or demodulation. Each reference signal is associated to an antenna port (AP) in the downlink of LTE system. Reference signals are predefined sequences which are allocated in predefined resource elements on the sub-frame.
In LTE Rel-8 DL transmission, cell-specific reference signals (CRS) are defined for both measurement and demodulation. The CRSs are broadcasted and common to all the UEs in the cell, and each CRS defines one antenna port. The number of antenna ports for eNB can be configured as 1, 2 and 4. The legacy control channel (PDCCH) uses CRS for modulation and decoding of control information in the system.
In LTE Rel-10 DL transmission, the RSs for measurement and demodulation are decoupled respectively to CSI reference signals (CSI-RS) and demodulation reference signals (DMRS). The CSI-RS which is used for measurement is UE-specific. Compared to the CRSs in LTE Rel-8, the CSI-RS has lower density in the frequency and time domain. The DMRS is used for PDSCH demodulation, which is also UE-specific. DMRS positions in sub-frames are always occupying the same positions. For some cases there are 12 resource elements used for DMRSs and for some case there are 24 resource elements are used. DMRS is used for modulation and decoding of newly designed ePDCCH.
The exact number of resource elements used for CSI-RS depends on the number of used antenna ports, sub-frame index and the number of used configurations. In a resource block pair there are up to 40 possible positions for CSI-RS as it is shown in FIG. 1. Generally a subset of those resource elements are used for desired UE.
For example, in the case of two antenna ports for a specific UE, CSI-RS resource elements of this UE occupy two consecutive reference symbols in a PRB pair separated by orthogonal cover code (OCC). This corresponds to one single configuration. Non-zero transmission power over these resource elements is assumed which enable the desired UE to perform its own measurement. A specific UE is also informed about CSI-RS configurations of other UEs. Zero-transmission power over resource elements allocated to other UEs is assumed. The combination of this information enables the desired UE to extract from the received signal, symbols containing its own data without ambiguity.
Signalling of CSI-RS is performed via higher layer signalling over PDSCH, being in that way dependent to PDCCH control channel One PDCCH (or ePDCCH) carries one DCI (Downlink Control Information) message. There are different formats of DCI in the system. In initial communication stage related to control channel a UE should detect related DCI format in the common search space successfully. Once the DCI format is successfully detected the UE can decode its PDSCH as resource allocation and MCS modulation related to PDSCH are included in DCI message. PDSCH is then used to convey higher layer signalling information to the UE. It means that no higher layer signalling is available prior to the correct detection of control channel.
Furthermore, new carrier type is defined as a carrier that is not backwards compatible to previous releases, comprising one or several characteristics of:                Not supporting transmission of the CRS;        Not supporting the use of CRS for demodulation;        Not supporting transmission of at least one of PDCCH, PCFICH, PHICH or PBCH;        Only supporting DM-RS based PDSCH demodulation;        Not supporting transmission of at least one of the PSS and the SSS; and        Supporting PDSCH being mapped from the first OFDM symbol in the subframe.        
As opposed to CRSs and DMRSs, whose resource elements are predefined and thus have known positions within each PRB pair, the CSI-RSs have UE-specific time-frequency positions. Moreover, according to LTE Rel-10 each UE is informed about both its own CSI-RS and the CSI-RS configurations of other UEs active in the same sub-frame in order to be able to correctly extract its own data from the PDSCH. Signalling of CSI-RS is performed using a bitmap via higher layer signalling over PDSCH being in that way dependent on the PDCCH. As the ePDCCH is also a downlink control channel, it is natural to expect that it can be used also to support higher-layer signalling of CSI-RS bitmap independent of PDCCH (legacy control channel).
At the initial stage related to control channel communication, the UE monitors only the ePDCCH common search space to get information about DCI format if there is no PDCCH available. At this stage there is no any information about CSI-RS bitmap available as ePDCCH has not been decoded yet. Without CSI-RS bit map information the UE can't decode ePDCCH correctly if there is CSI-RS located in the common search space. This is a chicken and egg problem. Therefore, even if the ePDCCH is available in the system, if the eCCEs are mapped to ePDCCH by rate matching the signalling of the CSI-RS bitmap to the UE still has to be supported by the PDCCH at the initial communication.
Another problem with rate matching around CSI-RS (i.e. mapping control channel around CSI-RS) is that for some configurations overhead of reference signals becomes considerable. This problem is valid for both UE-specific search space and common search space.
One proposal could be that the eCCE is rate matched with respect to DMRS (i.e. mapped around DMRS) but punctured by CSI-RS. The UE does not know the position of CSI-RS, so it performs the ePDCCH demodulation as if there is no CSI-RS in the ePDCCH PRB pairs (although the CSI-RS is actually completely transmitted). The transmitter adjusts the ePDCCH code rate (i.e. the aggregation level) to compensate the ePDCCH demodulation errors. The problem with this solution is that if CSI-RS overhead is considerable then coding rate is considerably reduced leading to a high overhead and un-efficient usage of ePDCCH resources which can lead to blocking of some UEs.
Hence one problem to solve is how to transmit ePDCCH with minimum overhead and independent of higher layer signalling. Two different approaches for mapping eCCEs into a PRB pair have been discussed in prior art.
The first approach, called rate matching consists of mapping eCCEs around current reference signals including CRS, DMRS and CSI-RS. In this approach the number of eCCEs in a RB pair is variable and depends on the reference signal overhead.
In the second approach, called eCCE puncturing the maximum number of eCCEs mapped in a PRB pair is fixed, and when there is a collision between reference signals and an eCCE, the eCCE is punctured and is not transmitted. In this latter approach it is assumed that eCCE puncturing is happening only when there is a collision with CRS and CSI-RS reference signals. More precisely, eCCEs are rate matched around DMRS reference signals. In order to keep the detection performance of eCCE acceptable, it is proposed to use higher aggregation levels. More precisely, in the second approach it is assumed that each eCCE or building structure has 36 resource elements and one, two four eCCEs could be aggregated into one PRB pair creating aggregation level of one, two or four. In this approach eCCE is rate matched with respect to DMRS but punctured by CSI-RS, CRS and legacy control channel. If this approach is used then ePDCCH transmission efficiency and performance might be highly affected. For example, if we consider the case where all CSI-RS resource elements are used in a PRB pair, the number of ePDCCH resource elements that are not punctured for aggregation level of one is 23 resource elements out of 36 resource elements. With such a high number of errors one might not be able to use aggregation level of one at all. This number is about 40 resource elements out of total 72, for aggregation level of two if we assume that link adaptation is used to enable the usage of aggregation level of two instead of one. It is even not sure that aggregation level of two could perform better with such a high error. Consequently, low aggregation levels are not transmitted at all. Using high aggregation levels instead of low aggregation levels is not efficient to allocate ePDCCH resources in different PRBs. Another drawback is that, even for high aggregation levels as receiver will attempt to blindly decode transmitted CSI-RS and CRS, there is a high probability of false detection which might deteriorate the overall performance of ePDCCH compared to PDCCH.
According to both prior art approaches described above CSI-RS reference signal transmission are maintained.
Another prior art consists of 3GPP specifications, where collision of CSI-RS for some of transport channels is considered. It is assumed that CSI-RS are not transmitted at all and are punctured in sub-frames containing paging message and sub-frames when transmission of CSI-RS would collide with transmission of synchronization signal and PBCH.